Kalper Construction Details
Pulling down a very rare model engine to its constituent parts is not a task to be taken lightly—even more so when the subject is rare, delicate, and replacement parts impossible to obtain! So we are greatly indebted to Jan Huning for the photos and detailed information on this page. Jan is no beginner to this sort of task, but I strongly advise those not experienced to think three times and have a nap before even considering it.
Adrian Dunca's description of the little British made Kalper 0.32 cc compression ignition engine hinted at what must be inside. And while he hit it largely on the head, there are aspects that only a full disassembly could disclose. The following text was provided by Jan.
Back in the mid '70's we (or rather, Dad) bought a Kalper from someone in the local model aircraft club. It had obviously been used, but was in good condition. I remember running it a few times back then, and how easy it was to operate, just as described in the article. I got the engine out a few months ago, and ran it again. It still worked, but the contra-piston was a bit loose, and tended to move up and down a little when warm, especially once a little fuel had leaked past and into the space between contra-piston and head, so it was difficult to restart. I recently got around to dismantling the engine and correcting the contra-piston fit, and took a few photos.
The engine came in a slightly tatty box, with a reprint of the L.H.Sparey Aeromodeller review. The review states that the contra-piston is a "special alloy". Well, it looks just like brass to me, and is quite soft. It appears to have been pressed from sheet about 1/32" thick, and is about 1/8" long. The fit was restored by gently expanding it a fraction, using a brass drift. There is now no leakage past it, it has the "correct" friction fit, and the engine ran very nicely yesterday.
Dismantling the engine was interesting. There are no slots or holes in the backplate or front screwed-in sections to allow any removal tool to be used. So I machined a piece of aluminium to a close fit over the exposed flange, which is barely 1/32 long, slit the tube, and then used the 3-jaw to tighten it onto the backplate flange. This gave enough grip, and spread the clamping force enough not to make any marks on the back-plate. The thread is left-handed. Similarly for the front section. In fact it is the same size thread front and back, probably tapped straight through in one go with the central channel for the conrod big end formed later. The thread is 40 TPI and presumably the left-hand thread is to prevent any tendency for the front section to unscrew when running. I found out that it was left handed when gently trying to turn both clockwise and anticlockwise, (a habit when threads are reluctant to turn) and was surprised that it moved further when turned clockwise.
The finned head screws onto the steel cylinder, which in turn screws into the crankcase (with right-hand threads!). The cylinder screws in from the top of the crankcase. The threads which hold the cylinder into the crankcase are at the bottom of the cylinder, with a plain unthreaded section above, around the exhaust, inlet and transfer ports. The threads at the top of the cylinder, for the finned head, are the same size as those at the bottom. The threads are 60 TPI, with a diameter over the crests of 0.316". The plain section in the middle is 0.322" diameter. At the top of the cylinder, above the threads for the head, is a short plain section of 0.295" diameter, leaving a wall thickness of only 0.022". At the bottom end of the cylinder there are two cut-outs, to clear the con-rod, making correct assembly and non-rotation of the liner in service absolutely critical
A flat has been ground at the front of the cylinder, below the transfer port, which together with a groove machined into the front of the crankcase creates the transfer passage. The cylinder is locked in place by the head, which screws against the top of the crankcase. So great care is needed to ensure the cylinder is in the correct orientation when crankcase, cylinder and head are all screwed together, otherwise the conrod hits the bottom edge of the cylinder. The piston needs to be fitted before the cylinder is screwed in, as there is insufficient space to assemble it from the bottom. The front screwed-in section with the crankshaft has to be fitted after the piston is in place, since the big end of the rod fits in a close fitting groove in the crankcase, minimising crankcase volume.
I was surprised at how rough some of the internal machining is. Neither the back face of the front section or the front face of the crankweb are relieved to form a thrust face, and the finish on both faces is poor. The thrust area is the complete crankweb area. Despite some of the rough internal machining, there are some nice details, such as a small flanged brass insert pressed in from the inside of the head for the compression screw, and a screwed-in brass insert in the back of the spinner nut. As stated in the article, all the threads are very fine. The compression screw, crankshaft thread and all the threaded connections on the intake assembly are 1/8" by 70 TPI. The photo here shows the Kalper along side the little French Moustique.
Jan Huning, July 2008
There's not much I can add to Jan's analysis, but I've never let a little detail like that stop me before. So notice the very long piston. This "feature" had been advocated by Lawerence Sparey in his pioneering 5 cc and ".8" cc designs. His published reasoning was that the long leakage path provided better compression retention than a shorter piston, especially for amateur constructors whose equipment and techniques may not be all that could be desired. This argument is flawed. It's more difficult for an amateur to hone/lap a long piston to perfect regularity than a short one without creating a barrel, and the extra size adds extra reciprocating mass, limiting performance. But his argument, forcefully made, held sway for some time and may have influenced the Kalper designer.
Second, the 1/8x70 thread is very unusual, but a possible explanation resides in the description of the makers: Seymour, Hylda & Co, Engineers' Instrument Makers. add to this the era in which the engine appeared, circa 1948. We can make an assumption that Seymore Hylda had not long before been engaged in war effort related work and their speciality of "Engineers' Instrument Makers" suggests they may have worked with some small, precision devices. Anyone want to bet they had a drawer full of 70 TPI taps and dies left over from making some piece of critical equipment that was no longer required?
And lastly: Warning, Warning, Will Robertson!
The Kalper cylinder is thin and hard. The pressed contra piston is rather soft, and as Jan's story illustrates, prone to wearing loose. We know of two instances of cracked Kalper cylinder liners. One by the insertion of a replacement contra made from cast-iron that was just a few tenths of a thou too large, resulting in a catastrophic and final "tink" sound. The other was most probably the result of a similar action. The relatively soft conra material in the original would reduce this danger, but I suspect it could still happen with an over-zealous application of the method Jan used to expand his contra piston. You Have Been Warned!